Monitoring watercourse vegetation, a synecological approach to dynamic gradients

Changes in vegetation under reduced control measures over 3 to 5 years in watercourses in a rural environment in The Netherlands were evaluated. A method to deal with slow changes on a steep gradient is presented. The gradient with various vegetation types between the middle of the watercourse and the bank-top was split up into zones. Species composition of each zone was evaluated using literature on syntaxonomy. Cover of character species, multiplied by the width of the zones, was used to quantify the contribution of various syntaxa in the vegetation. Changes in these contribution data were used to evaluate changes over the years. The method was applied to two experiments in which cleaning frequency was reduced. Submerged vegetation of 'Callitriche-Ranunculetum penicillati' in one and of 'Potamogetonetalia pectinati' in the other case, hardly changed. Emergent vegetation of 'Nasturtio-Glycerietalia' or 'Sparganio-Glycerietum fiuitans' tended to expand into the submerged zone. Bank vegetation began to show signs of development into ruderal vegetation, as a shift from 'Molinio-Arrhenatheretea' into 'Artemisietea' was detected. The method allowed the conclusion that conditions were too eutrophic in both experiments for a diverse brook vegetation development without additional habitat improvement.